In recent years, electric power steering apparatus have been popularly used to reduce steering force to be manually applied to a steering wheel by a human operator or driver and thereby achieve a comfortable steering feel. Such electric power steering apparatus are designed to produce steering assist torque, by means of an electric steering assisting motor, in accordance with steering force applied to the steering wheel, and then transmit the steering assist torque to a rack and pinion mechanism of a steering system. One example of the conventional electric power steering apparatus is proposed by Japanese Patent Laid-Open Publication No. HEI-9-30432. The proposed electric power steering apparatus will be outlined below with reference to FIGS. 8, 9 and 10.
In the conventional electric power steering apparatus proposed in the No. HEI-9-30432 laid-open publication, as shown in FIG. 8, a steering torque sensor 102 detects steering torque applied to the steering wheel 101, a control device 103 generates a control signal on the basis of a torque detection signal generated by the torque sensor 102, and an electric steering assisting motor 104 generates steering assist torque on the basis of the control signal generated by the control device. The thus-generated steering assist torque is transmitted to a pinion shaft 106 via a worm gear mechanism 105, and then delivered from the pinion shaft 106 to a rack and pinion mechanism 108 of a steering system 107. Steerable road wheels 111 are steered by composite torque, i.e. a combination of the driver-applied steering torque and steering assist torque from the motor 104, via a rack shaft 109.
As illustrated in FIG. 9, the worm gear mechanism 105 includes a worm 121 formed on a rotation shaft 104a of the steering assisting motor 104, and a worm wheel 122 meshing with the worm 121 and coupled to the pinion shaft 106. Reference numeral 112 represents a housing.
In the worm gear mechanism 105 of the conventional electric power steering apparatus, the lead angle of threads of the worm 121 is set to be slightly greater than the friction angle of the thread surfaces; the reason is to allow the worm 121 to be rotated by the worm wheel 122. When the electric steering assisting motor 104 is not in operation or deactivated, i.e. when the motor 104 is not producing steering assist torque, the steerable wheels 111 can be steered freely only by the driver-applied steering torque. During that time, the rotor 104b of the assisting motor 104 is rotated by the driver-applied steering torque via the pinion shaft 106, worm wheel 122, worm 121 and motor's rotation shaft 104a. 
It has been conventional to construct such a worm gear mechanism 105 of the electric power steering apparatus 100 in a manner as illustrated in FIG. 10. FIG. 10 shows how the worm 121 meshes with the worm wheel 122.
The worm 121 has a pitch circle diameter d1, while the worm wheel 122 has a pitch circle diameter d2. The worm 121 is a so-called “double-thread worm”. The lead Le of the worm wheel 122 is twice as great as the pitch pi (Le=2×pi). Here, the “lead” means an amount of an axial advance per helical turn of threads. The worm 121 and worm wheel 122 each have a pressure angle α3.
In the electric power steering apparatus, when the electric steering assisting motor 104 is not in operation, i.e. when the motor 104 is not producing steering assist torque, the driver-applied steering torque is subjected to undesirable influences of inertia of the motor 104 and meshing resistance of the worm gear mechanism 105. Thus, it is preferable that such influences be as small as possible. Particularly, the steering torque would be significantly influenced when variation has occurred in meshing resistance of the worm wheel 122 against the worm 121, i.e. frictional force of the sliding surface of the worm wheel 122 against the sliding surface of the worm 121, due to insufficient working accuracy, such as unevenness in finished roughness across the sliding surfaces. Because variation in the steering torque would prevent smooth maneuver or operation of the steering wheel 101, it is preferable to minimize the steering torque variation, in order to improve the steering feel.
Further, in the worm gear mechanism 105 employing the double-thread worm 121, first and second threads of the worm 121 simultaneously mesh with at least two teeth of the worm wheel 122; namely, in this case, the contact ratio is “2”. Therefore, theoretically, smooth mesh can be expected which may produce relatively small meshing variation, and influences of the meshing resistance of the worm gear mechanism 105 can be lessened effectively. In fact, however, variation would occur in the meshing resistance of the worm wheel 122 against the worm 121, i.e. the frictional force between the sliding surfaces, due to influences of insufficient working accuracy etc., as noted above. Particularly, because the worm 121 is a double-thread worm, there is a need to make the accuracy of the pitch pi of the first thread and the accuracy of the pitch pi of the second thread as equal as possible. If the accuracy of the pitch pi of the first thread and the accuracy of the pitch pi of the second thread is not equal to each other, then the frictional force between the sliding surfaces would vary, which undesirably influences the steering torque.
Namely, the undesirable influences of the accuracy of the pitches pi are substantial and may sometimes overweigh the benefits of the enhanced contact ratio. Because variation in the steering torque would disturb smooth maneuver of the steering wheel 101, it is preferable to minimize the steering torque variation, in order to improve the steering feel.
For reduction in the variation amount of the frictional force between the sliding surfaces, it may be considered useful to enhance the working and assembling accuracy of the worm 121 and worm wheel 122. However, merely enhancing such working and assembling accuracy would increase the total number of necessary control steps and hence necessary costs, and is not an advisable approach; thus, there is a room for further improvement.
Therefore, in the electric power steering apparatus, it is desirable to even further improve the steering feel by reducing the amount of variation in steering torque that would be produced due to variation in the meshing resistance (frictional force) of the worm gear mechanism.